2005 radiative forcings and uncertainties as estimated by the IPCC.
Atmospheric aerosols affect the climate of the earth by changing the amount of incoming solar radiation
and outgoing terrestrial longwave radiation retained in the earth's
system. This occurs through several distinct mechanisms which are split
into direct, indirect
and semi-direct aerosol effects. The aerosol climate effects are the
biggest source of uncertainty in future climate predictions. The Intergovernmental Panel on Climate Change, Third Assessment Report, says: While the radiative forcing due to greenhouse gases
may be determined to a reasonably high degree of accuracy... the
uncertainties relating to aerosol radiative forcings remain large, and
rely to a large extent on the estimates from global modelling studies
that are difficult to verify at the present time.
Aerosol radiative effects
Global aerosol optical thickness. The aerosol scale (yellow to dark reddish-brown) indicates the relative amount of particles that absorb sunlight.
Direct effect
Particulates in the air causing shades of grey and pink in Mumbai during sunset
The direct aerosol effect consists of any direct interaction of
radiation with atmospheric aerosols, such as absorption or scattering.
It affects both short and longwave radiation to produce a net negative
radiative forcing. The magnitude of the resultant radiative forcing due to the direct effect of an aerosol is dependent on the albedo
of the underlying surface, as this affects the net amount of radiation
absorbed or scattered to space. e.g. if a highly scattering aerosol is
above a surface of low albedo it has a greater radiative forcing than if
it was above a surface of high albedo. The converse is true of
absorbing aerosol, with the greatest radiative forcing arising from a
highly absorbing aerosol over a surface of high albedo. The direct aerosol effect is a first order effect and is therefore classified as a radiative forcing by the IPCC. The interaction of an aerosol with radiation is quantified by the single-scattering albedo (SSA), the ratio of scattering alone to scattering plus absorption (extinction)
of radiation by a particle. The SSA tends to unity if scattering
dominates, with relatively little absorption, and decreases as
absorption increases, becoming zero for infinite absorption. For
example, the sea-salt aerosol has an SSA of 1, as a sea-salt particle
only scatters, whereas soot has an SSA of 0.23, showing that it is a
major atmospheric aerosol absorber.
Indirect effect
The
Indirect aerosol effect consists of any change to the earth's radiative
budget due to the modification of clouds by atmospheric aerosols, and
consists of several distinct effects. Cloud droplets form onto
pre-existing aerosol particles, known as cloud condensation nuclei (CCN).
For any given meteorological conditions, an increase in CCN leads
to an increase in the number of cloud droplets. This leads to more
scattering of shortwave radiation i.e. an increase in the albedo of the
cloud, known as the Cloud albedo effect, First indirect effect or Twomey effect. Evidence supporting the cloud albedo effect has been observed from the effects of ship exhaust plumes and biomass burning
on cloud albedo compared to ambient clouds. The Cloud albedo aerosol
effect is a first order effect and therefore classified as a radiative
forcing by the IPCC.
An increase in cloud droplet number due to the introduction of
aerosol acts to reduce the cloud droplet size, as the same amount of
water is divided into more droplets. This has the effect of suppressing
precipitation, increasing the cloud lifetime, known as the cloud
lifetime aerosol effect, second indirect effect or Albrecht effect. This has been observed as the suppression of drizzle in ship exhaust plume compared to ambient clouds, and inhibited precipitation in biomass burning plumes. This cloud lifetime effect is classified as a climate feedback (rather than a radiative forcing) by the IPCC due to the interdependence between it and the hydrological cycle. However, it has previously been classified as a negative radiative forcing.
Semi-direct effect
The
Semi-direct effect concerns any radiative effect caused by absorbing
atmospheric aerosol such as soot, apart from direct scattering and
absorption, which is classified as the direct effect. It encompasses
many individual mechanisms, and in general is more poorly defined and
understood than the direct and indirect aerosol effects. For instance,
if absorbing aerosols are present in a layer aloft in the atmosphere,
they can heat surrounding air which inhibits the condensation of water
vapour, resulting in less cloud formation.
Additionally, heating a layer of the atmosphere relative to the surface
results in a more stable atmosphere due to the inhibition of
atmospheric convection. This inhibits the convective uplift of moisture,
which in turn reduces cloud formation. The heating of the atmosphere
aloft also leads to a cooling of the surface, resulting in less
evaporation of surface water. The effects described here all lead to a
reduction in cloud cover i.e. an increase in planetary albedo. The
semi-direct effect classified as a climate feedback) by the IPCC due to the interdependence between it and the hydrological cycle. However, it has previously been classified as a negative radiative forcing.
Roles of different aerosol species
Sulfate aerosol
Sulfate aerosol has two main effects, direct and indirect. The direct effect, via albedo, is a cooling effect that slows the overall rate of global warming: the IPCC's best estimate of the radiative forcing is −0.4 watts per square meter with a range of −0.2 to −0.8 W/m²
but there are substantial uncertainties. The effect varies strongly
geographically, with most cooling believed to be at and downwind of
major industrial centres. Modern climate models addressing the attribution of recent climate change
take into account sulfate forcing, which appears to account (at least
partly) for the slight drop in global temperature in the middle of the
20th century. The indirect effect (via the aerosol acting as cloud
condensation nuclei, CCN, and thereby modifying the cloud properties -albedo and lifetime-) is more uncertain but is believed to be a cooling.
Black carbon
Black
carbon (BC), or carbon black, or elemental carbon (EC), often called
soot, is composed of pure carbon clusters, skeleton balls and buckyballs,
and is one of the most important absorbing aerosol species in the
atmosphere. It should be distinguished from organic carbon (OC):
clustered or aggregated organic molecules on their own or permeating an
EC buckyball. BC from fossil fuels is estimated by the IPCC in the
Fourth Assessment Report of the IPCC, 4AR, to contribute a global mean
radiative forcing of +0.2 W/m² (was +0.1 W/m² in the Second Assessment
Report of the IPCC, SAR), with a range +0.1 to +0.4 W/m². Bond et al.,
however, states that "the best estimate for the industrial-era (1750 to
2005) direct radiative forcing of atmospheric black carbon is +0.71 W/m²
with 90% uncertainty bounds of (+0.08, +1.27) W/m²" with "total direct
forcing by all black carbon sources, without subtracting the
preindustrial background, is estimated as +0.88 (+0.17, +1.48) W/m²"
Instances of aerosol affecting climate
Solar radiation reduction due to volcanic eruptions
Volcanoes are a large natural source of aerosol and have been linked
to changes in the earth's climate often with consequences for the human
population. Eruptions linked to changes in climate include the 1600
eruption of Huaynaputina which was linked to the Russian famine of 1601 - 1603, leading to the deaths of two million, and the 1991 eruption of Mount Pinatubo which caused a global cooling of approximately 0.5 °C lasting several years.
Research tracking the effect of light-scattering aerosols in the
stratosphere during 2000 and 2010 and comparing its pattern to volcanic
activity show a close correlation. Simulations of the effect of
anthropogenic particles showed little influence at present levels.
Aerosols are also thought to affect weather and climate on a
regional scale. The failure of the Indian Monsoon has been linked to the
suppression of evaporation of water from the Indian Ocean due to the
semi-direct effect of anthropogenic aerosol.
Recent studies of the Sahel drought and major increases since 1967 in rainfall over the Northern Territory, Kimberley, Pilbara and around the Nullarbor Plain have led some scientists to conclude that the aerosol haze over South and East Asia has been steadily shifting tropical rainfall in both hemispheres southward.
The latest studies of severe rainfall decline over southern Australia since 1997
have led climatologists there to consider the possibility that these
Asian aerosols have shifted not only tropical but also midlatitude
systems southward.
It was the first supernova that modern astronomers were able to study in great detail, and its observations have provided much insight into core-collapse supernovae.
SN 1987A provided the first opportunity to confirm by direct
observation the radioactive source of the energy for visible light
emissions, by detecting predicted gamma-ray line radiation from two of
its abundant radioactive nuclei. This proved the radioactive nature of
the long-duration post-explosion glow of supernovae.
Four days after the event was recorded, the progenitor star was tentatively identified as Sanduleak −69 202 (Sk -69 202), a blue supergiant.
After the supernova faded, that identification was definitely confirmed
by Sk −69 202 having disappeared. This was an unexpected identification,
because models of high mass stellar evolution at the time did not predict that blue supergiants are susceptible to a supernova event.
Some models of the progenitor attributed the color to its
chemical composition rather than its evolutionary state, particularly
the low levels of heavy elements, among other factors. There was some speculation that the star might have merged with a companion star before the supernova.
However, it is now widely understood that blue supergiants are natural
progenitors of some supernovae, although there is still speculation that
the evolution of such stars could require mass loss involving a binary
companion.
Neutrino emissions
Remnant of SN 1987A seen in light overlays of different spectra. ALMA data (radio, in red) shows newly formed dust in the center of the remnant. Hubble (visible, in green) and Chandra (X-ray, in blue) data show the expanding shock wave.
Approximately two to three hours before the visible light from SN 1987A reached Earth, a burst of neutrinos was observed at three neutrino observatories.
This was likely due to neutrino emission, which occurs simultaneously
with core collapse, but before visible light was emitted. Visible light
is transmitted only after the shock wave reaches the stellar surface. At 07:35 UT, Kamiokande II detected 12 antineutrinos; IMB, 8 antineutrinos; and Baksan, 5 antineutrinos; in a burst lasting less than 13 seconds. Approximately three hours earlier, the Mont Blancliquid scintillator detected a five-neutrino burst, but this is generally not believed to be associated with SN 1987A.
The Kamiokande II detection, which at 12 neutrinos had the
largest sample population, showed the neutrinos arriving in two distinct
pulses. The first pulse started at 07:35:35 and comprised 9 neutrinos,
all of which arrived over a period of 1.915 seconds. A second pulse of
three neutrinos arrived between 9.219 and 12.439 seconds after the first
neutrino was detected, for a pulse duration of 3.220 seconds.
Although only 25 neutrinos were detected during the event, it was
a significant increase from the previously observed background level.
This was the first time neutrinos known to be emitted from a supernova
had been observed directly, which marked the beginning of neutrino astronomy.
The observations were consistent with theoretical supernova models in
which 99% of the energy of the collapse is radiated away in the form of
neutrinos. The observations are also consistent with the models' estimates of a total neutrino count of 1058 with a total energy of 1046 joules, i.e. a mean value of some dozens of MeV per neutrino.
The neutrino measurements allowed upper bounds on neutrino mass
and charge, as well as the number of flavors of neutrinos and other
properties. For example, the data show that within 5% confidence, the rest mass of the electron neutrino is at most 16 eV/c2,
1/30,000 the mass of an electron. The data suggest that the total
number of neutrino flavors is at most 8 but other observations and
experiments give tighter estimates. Many of these results have since
been confirmed or tightened by other neutrino experiments such as more
careful analysis of solar neutrinos and atmospheric neutrinos as well as
experiments with artificial neutrino sources.
Missing neutron star
The bright ring around the central region of the exploded star is composed of ejected material.
SN 1987A appears to be a core-collapse supernova, which should result in a neutron star given the size of the original star.
The neutrino data indicate that a compact object did form at the star's
core. However, since the supernova first became visible, astronomers
have been searching for the collapsed core but have not detected it. The
Hubble Space Telescope
has taken images of the supernova regularly since August 1990, but, so
far, the images have shown no evidence of a neutron star. A number of
possibilities for the 'missing' neutron star are being considered. The first is that the neutron star is enshrouded in dense dust clouds so that it cannot be seen. Another is that a pulsar
was formed, but with either an unusually large or small magnetic field.
It is also possible that large amounts of material fell back on the
neutron star, so that it further collapsed into a black hole.
Neutron stars and black holes often give off light as material falls
onto them. If there is a compact object in the supernova remnant, but no
material to fall onto it, it would be very dim and could therefore
avoid detection. Other scenarios have also been considered, such as
whether the collapsed core became a quark star.
Light curve
Much of the light curve, or graph of luminosity as a function of time, after the explosion of a type II supernova such as SN 1987A is provided its energy by radioactive decay.
Although the luminous emission consists of optical photons, it is the
radioactive power absorbed that keeps the remnant hot enough to radiate
light. Without radioactive heat it would quickly dim. The radioactive
decay of 56Ni through its daughters 56Co to 56Fe produces gamma-ray photons
that are absorbed and dominate the heating and thus the luminosity of
the ejecta at intermediate times (several weeks) to late times (several
months). Energy for the peak of the light curve of SN1987A was provided by the decay of 56Ni to 56Co (half life of 6 days) while energy for the later light curve in particular fit very closely with the 77.3-day half-life of 56Co decaying to 56Fe. Later measurements by space gamma-ray telescopes of the small fraction of the 56Co and 57Co gamma rays that escaped the SN1987A remnant without absorption confirmed earlier predictions that those two radioactive nuclei were the power source.
Because the 56Co in SN1987A has now completely
decayed, it no longer supports the luminosity of the SN 1987A ejecta.
That is currently powered by the radioactive decay of 44Ti
with a half life of about 60 years. With this change, X-rays produced
by the ring interactions of the ejecta began to contribute significantly
to the total light curve. This was noticed by the Hubble Space
Telescope as a steady increase in luminosity 10,000 days after the event
in the blue and red spectral bands. X-ray lines 44Ti observed by the INTEGRAL space X-ray telescope showed that the total mass of radioactive 44Ti synthesized during the explosion was 3.1 ± 0.8×10−4M☉.
Observations of the radioactive power from their decays in the 1987A light curve have measured accurate total masses of the 56Ni, 57Ni, and 44Ti
created in the explosion, which agree with the masses measured by
gamma-ray line space telescopes and provides nucleosynthesis constraints
on the computed supernova model.
Interaction with circumstellar material
The expanding ring-shaped remnant of SN 1987A and its interaction with its surroundings, seen in X-ray and visible light.
Sequence of HST images from 1994 to 2009, showing the collision of the expanding remnant with a ring of material ejected by the progenitor 20,000 years before the supernova
The three bright rings around SN 1987A that were visible after a few
months in images by the Hubble Space Telescope are material from the stellar wind
of the progenitor. These rings were ionized by the ultraviolet flash
from the supernova explosion, and consequently began emitting in various
emission lines. These rings did not "turn on" until several months
after the supernova; the turn-on process can be very accurately studied
through spectroscopy. The rings are large enough that their angular size
can be measured accurately: the inner ring is 0.808 arcseconds in
radius. The time light traveled to light up the inner ring gives its
radius of 0.66 (ly) light years.
Using this as the base of a right angle triangle and the angular size
as seen from the Earth for the local angle, one can use basic
trigonometry to calculate the distance to SN 1987A, which is about
168,000 light-years.
The material from the explosion is catching up with the material
expelled during both its red and blue supergiant phases and heating it,
so we observe ring structures about the star.
Around 2001, the expanding (>7000 km/s) supernova ejecta
collided with the inner ring. This caused its heating and the generation
of x-rays—the x-ray flux from the ring increased by a factor of three
between 2001 and 2009. A part of the x-ray radiation, which is absorbed
by the dense ejecta close to the center, is responsible for a comparable
increase in the optical flux from the supernova remnant in 2001–2009.
This increase of the brightness of the remnant reversed the trend
observed before 2001, when the optical flux was decreasing due to the
decaying of 44Ti isotope.
A study reported in June 2015, using images from the Hubble Space Telescope and the Very Large Telescope
taken between 1994 and 2014, shows that the emissions from the clumps
of matter making up the rings are fading as the clumps are destroyed by
the shock wave. It is predicted the ring will fade away between 2020 and
2030. These findings are also supported by the results of a
three-dimensional hydrodynamic model which describes the interaction of
the blast wave with the circumstellar nebula.
The model also shows that X-ray emission from ejecta heated up by the
shock will be dominant very soon, after the ring will fade away. As the
shock wave passes the circumstellar ring it will trace the history of
mass loss of the supernova's progenitor and provide useful information
for discriminating among various models for the progenitor of SN 1987A.
In 2018, radio observations from the interaction between the
circumstellar ring of dust and the shockwave has confirmed the shockwave
has now left the circumstellar material. It also shows that the speed
of the shockwave, which slowed down to 2,300 km/s while interacting with
the dust in the ring, has now re-accelerated to 3,600 km/s.
Condensation of warm dust in the ejecta
Images
of the SN 1987A debris obtained with the instruments T-ReCS at the 8-m
Gemini telescope and VISIR at one of the four VLT. Dates are indicated.
An HST image is inserted at the bottom right (credits Patrice Bouchet,
CEA-Saclay)
Soon after the SN 1987A outburst, three major groups embarked in a photometric monitoring of the supernova: SAAO, CTIO, and ESO. In particular, the ESO team reported an infrared excess
which became apparent beginning less than one month after the explosion
(March 11, 1987). Three possible interpretations for it were discussed
in this work: the infrared echo hypothesis was discarded, and thermal emission
from dust that could have condensed in the ejecta was favoured (in
which case the estimated temperature at that epoch was ~ 1250 K, and the
dust mass was approximately 6.6×10−7M☉). The possibility that the IR excess could be produced by optically thick free-free emission
seemed unlikely because the luminosity in UV photons needed to keep the
envelope ionized was much larger than what was available, but it was
not ruled out in view of the eventuality of electron scattering, which
had not been considered.
However, none of these three groups had sufficiently convincing
proofs to claim for a dusty ejecta on the basis of an IR excess alone.
Distribution of the dust inside the SN 1987A ejecta, as from the Lucy et al.'s model built at ESO
An independent Australian team advanced several argument in favour of an echo interpretation. This seemingly straightforward interpretation of the nature of the IR emission was challenged by the ESO group and definitively ruled out after presenting optical evidence for the presence of dust in the SN ejecta.
To discriminate between the two interpretations, they considered the
implication of the presence of an echoing dust cloud on the optical
light curve, and on the existence of diffuse optical emission around the
SN.
They concluded that the expected optical echo from the cloud should be
resolvable, and could be very bright with an integrated visual
brightness of magnitude 10.3 around day 650. However, further optical observations, as expressed in SN light curve, showed no inflection
in the light curve at the predicted level. Finally, the ESO team
presented a convincing clumpy model for dust condensation in the ejecta.
Although it had been thought more than 50 years ago that dust could form in the ejecta of a core-collapse supernova, which in particular could explain the origin of the dust seen in young galaxies,
that was the first time that such a condensation was observed. If SN
1987A is a typical representative of its class then the derived mass of
the warm dust formed in the debris of core collapse supernovae is not
sufficient to account for all the dust observed in the early universe.
However, a much larger reservoir of ~0.25 solar mass of colder dust (at
~26 K) in the ejecta of SN 1987A was found with the Hershel infrared space telescope in 2011 and confirmed by ALMA later on (in 2014).
ALMA observations
Following the confirmation of a large amount of cold dust in the ejecta,
ALMA has continued observing SN 1987A. Synchrotron radiation due to
shock interaction in the equatorial ring has been measured. Cold
(20–100K) carbon monoxide (CO) and silicate molecules (SiO) were
observed. The data show that CO and SiO distributions are clumpy, and
that different nucleosynthesis products (C, O and Si) are located in
different places of the ejecta, indicating the footprints of the stellar
interior at the time of the explosion.
The known history of supernova observation goes back to 185 AD, when supernovaSN 185 appeared, the oldest appearance of a supernova recorded by humankind. Several additional supernovae within the Milky Way galaxy have been recorded since that time, with SN 1604 being the most recent supernova to be observed in this galaxy.
Since the development of the telescope,
the field of supernova discovery has expanded to other galaxies. These
occurrences provide important information on the distances of galaxies.
Successful models of supernova behavior have also been developed, and
the role of supernovae in the star formation process is now increasingly
understood.
Early history
The guest star reported by Chinese astronomers in 1054 is identified as SN 1054. The highlighted passages refer to the supernova.
The supernova explosion that formed the Vela Supernova Remnant most likely occurred 10,000–20,000 years ago. In 1976, NASA astronomers suggested that inhabitants of the southern hemisphere may have witnessed this explosion and recorded it symbolically. A year later, archaeologist George Michanowsky recalled some incomprehensible ancient markings in Bolivia that were left by Native Americans.
The carvings showed four small circles flanked by two larger circles.
The smaller circles resemble stellar groupings in the constellations Vela and Carina. One of the larger circles may represent the star Capella.
Another circle is located near the position of the supernova remnant,
George Michanowsky suggested this may represent the supernova explosion
as witnessed by the indigenous residents.
In 185 CE, Chinese astronomers
recorded the appearance of a bright star in the sky, and observed that
it took about eight months to fade from the sky. It was observed to
sparkle like a star and did not move across the heavens like a comet.
These observations are consistent with the appearance of a supernova,
and this is believed to be the oldest confirmed record of a supernova
event by humankind. SN 185 may have also possibly been recorded in Roman literature, though no records have survived.
The gaseous shell RCW 86 is suspected as being the remnant of this
event, and recent X-ray studies show a good match for the expected age.
In 393 CE, the Chinese recorded the appearance of another "guest star", SN 393, in the modern constellation of Scorpius. Additional unconfirmed supernovae events may have been observed in 369 CE, 386 CE,
437 CE, 827 CE and 902 CE.
However these have not yet been associated with a supernova remnant,
and so they remain only candidates. Over a span of about 2,000 years,
Chinese astronomers recorded a total of twenty such candidate events,
including later explosions noted by Islamic, European, and possibly
Indian and other observers.
The supernova SN 1006 appeared in the southern constellation of Lupus
during the year 1006 CE. This was the brightest recorded star ever to
appear in the night sky, and its presence was noted in China, Egypt, Iraq, Italy, Japan and Switzerland. It may also have been noted in France, Syria, and North America. Egyptian physician, astronomer and astrologer Ali ibn Ridwan
gave the brightness of this star as one-quarter the brightness of the
Moon. Modern astronomers have discovered the faint remnant of this
explosion and determined that it was only 7,100 light-years from the Earth.
Supernova SN 1054 was another widely observed event, with Arab, Chinese, and Japanese astronomers recording the star's appearance in 1054 CE. It may also have been recorded by the Anasazi as a petroglyph. This explosion appeared in the constellation of Taurus, where it produced the Crab Nebula remnant. At its peak, the luminosity of SN 1054 may have been four times as bright as Venus, and it remained visible in daylight for 23 days and was visible in the night sky for 653 days.
There are fewer records of supernova SN 1181, which occurred in the constellation Cassiopeia just over a century after SN 1054. It was noted by Chinese and Japanese astronomers, however. The pulsar3C58 may be the stellar relic from this event.
The Danish astronomer Tycho Brahe was noted for his careful observations of the night sky from his observatory on the island of Hven. In 1572 he noted the appearance of a new star, also in the constellation Cassiopeia. Later called SN 1572, this
supernova was associated with a remnant during the 1960s.
A common belief in Europe during this period was the Aristotelian idea that the world beyond the Moon and planets was immutable.
So observers argued that the phenomenon was something in the Earth's
atmosphere. However Tycho noted that the object remained stationary from
night to night—never changing its parallax—so it must lie far away. He published his observations in the small book De nova et nullius aevi memoria prius visa stella (Latin for "Concerning the new and previously unseen star") in 1573. It is from the title of this book that the modern word nova for cataclysmic variable stars is derived.
The most recent supernova to be seen in the Milky Way galaxy was SN 1604, which was observed October 9, 1604. Several people, including Johannes van Heeck, noted the sudden appearance of this star, but it was Johannes Kepler who became noted for his systematic study of the object. He published his observations in the work De Stella nova in pede Serpentarii.
Galileo,
like Tycho before him, tried in vain to measure the parallax of this
new star, and then argued against the Aristotelian view of an immutable
heavens. The remnant of this supernova was identified in 1941 at the Mount Wilson Observatory.
Telescope observation
The
true nature of the supernova remained obscure for some time. Observers
slowly came to recognize a class of stars that undergo long-term
periodic fluctuations in luminosity. Both John Russell Hind in 1848 and Norman Pogson
in 1863 had charted stars that underwent sudden changes in brightness.
However, these received little attention from the astronomical
community. Finally, in 1866, English astronomer William Huggins
made the first spectroscopic observations of a nova, discovering lines
of hydrogen in the unusual spectrum of the recurrent nova T Coronae Borealis. Huggins proposed a cataclysmic explosion as the underlying mechanism, and his efforts drew interest from other astronomers.
Animation
showing the sky position of supernovae discovered since 1885. Some
recent survey contributions are highlighted in color.
In 1885, a nova-like outburst was observed in the direction of the Andromeda Galaxy by Ernst Hartwig in Estonia. S Andromedae increased to 6th magnitude, outshining the entire nucleus of the galaxy, then faded in a manner much like a nova. In 1917, George W. Ritchey
measured the distance to the Andromeda Galaxy and discovered it lay
much farther than had previously been thought. This meant that S
Andromedae, which did not just lie along the line of sight to the galaxy
but had actually resided in the nucleus, released a much greater amount
of energy than was typical for a nova.
Early work on this new category of nova was performed during the 1930s by Walter Baade and Fritz Zwicky at Mount Wilson Observatory.
They identified S Andromedae, what they considered a typical supernova,
as an explosive event that released radiation approximately equal to
the Sun's total energy output for 107 years. They decided to
call this new class of cataclysmic variables super-novae, and postulated
that the energy was generated by the gravitational collapse of ordinary
stars into neutron stars. The name super-novae was first used in a 1931 lecture at Caltech by Zwicky, then used publicly in 1933 at a meeting of the American Physical Society. By 1938, the hyphen had been lost and the modern name was in use.
Although supernovae are relatively rare events, occurring on average about once every 50 years in the Milky Way,
observations of distant galaxies allowed supernovae to be discovered
and examined more frequently. The first supernova detection patrol was
begun by Zwicky in 1933. He was joined by Josef J. Johnson from Caltech in 1936. Using a 45-cm Schmidt telescope at Palomar observatory,
they discovered twelve new supernovae within three years by comparing
new photographic plates to reference images of extragalactic regions.
In 1938, Walter Baade became the first astronomer to identify a nebula as a supernova remnant when he suggested that the Crab Nebula was the remains of SN 1054. He noted that, while it had the appearance of a planetary nebula, the measured velocity of expansion was much too large to belong to that classification.
During the same year, Baade first proposed the use of the Type Ia
supernova as a secondary distance indicator. Later, the work of Allan Sandage and Gustav Tammann helped refine the process so that Type Ia supernovae became a type of standard candle for measuring large distances across the cosmos.
The first spectral classification of these distant supernovae was performed by Rudolph Minkowski
in 1941. He categorized them into two types, based on whether or not
lines of the element hydrogen appeared in the supernova spectrum.
Zwicky later proposed additional types III, IV, and V, although these
are no longer used and now appear to be associated with single peculiar
supernova types. Further sub-division of the spectra categories resulted
in the modern supernova classification scheme.
In the aftermath of the Second World War, Fred Hoyle
worked on the problem of how the various observed elements in the
universe were produced. In 1946 he proposed that a massive star could
generate the necessary thermonuclear reactions, and the nuclear
reactions of heavy elements were responsible for the removal of energy
necessary for a gravitational collapse to occur. The collapsing star
became rotationally unstable, and produced an explosive expulsion of
elements that were distributed into interstellar space. The concept that rapid nuclear fusion was the source of energy for a supernova explosion was developed by Hoyle and William Fowler during the 1960s.
The first computer-controlled search for supernovae was begun in the 1960s at Northwestern University. They built a 24-inch telescope at Corralitos Observatory in New Mexico
that could be repositioned under computer control. The telescope
displayed a new galaxy each minute, with observers checking the view on a
television screen. By this means, they discovered 14 supernovae over a
period of two years.
1970–1999
The modern standard model for Type Ia supernovae
explosions is founded on a proposal by Whelan and Iben in 1973, and is
based upon a mass-transfer scenario to a degenerate companion star. In particular, the light curve of SN1972e in NGC 5253,
which was observed for more than a year, was followed long enough to
discover that after its broad "hump" in brightness, the supernova faded
at a nearly constant rate of about 0.01 magnitudes per day. Translated to another system of units, this is nearly the same as the decay rate of cobalt-56 (56Co), whose half-life is 77 days. The degenerate explosion model predicts the production of about a solar mass of nickel-56 (56Ni) by the exploding star. The 56Ni decays with a half-life of 6.8 days to 56Co,
and the decay of the nickel and cobalt provides the energy radiated
away by the supernova late in its history. The agreement in both total
energy production and the fade rate between the theoretical models and
the observations of 1972e led to rapid acceptance of the
degenerate-explosion model.
Through observation of the light curves of many Type Ia
supernovae, it was discovered that they appear to have a common peak
luminosity.
By measuring the luminosity of these events, the distance to their host
galaxy can be estimated with good accuracy. Thus this category of
supernovae has become highly useful as a standard candle
for measuring cosmic distances. In 1998, the High-Z Supernova Search
and the Supernova Cosmology Project discovered that the most distant
Type Ia supernovae appeared dimmer than expected. This has provided
evidence that the expansion of the universe may be accelerating.
Although no supernova has been observed in the Milky Way since
1604, it appears that a supernova exploded in the constellation
Cassiopeia about 300 years ago, around the year 1667 or 1680. The
remnant of this explosion, Cassiopeia A—is
heavily obscured by interstellar dust, which is possibly why it did not
make a notable appearance. However it can be observed in other parts of
the spectrum, and it is currently the brightest radio source beyond our
solar system.
In 1987, Supernova 1987A in the Large Magellanic Cloud was observed within hours of its start. It was the first supernova to be detected through its neutrino emission and the first to be observed across every band of the electromagnetic spectrum.
The relative proximity of this supernova has allowed detailed
observation, and it provided the first opportunity for modern theories
of supernova formation to be tested against observations.
The rate of supernova discovery steadily increased throughout the twentieth century.
In the 1990s, several automated supernova search programs were
initiated. The Leuschner Observatory Supernova Search program was begun
in 1992 at Leuschner Observatory. It was joined the same year by the Berkeley Automated Imaging Telescope program. These were succeeded in 1996 by the Katzman Automatic Imaging Telescope at Lick Observatory,
which was primarily used for the Lick Observatory Supernova Search
(LOSS). By 2000, the Lick program resulted in the discovery of 96
supernovae, making it the world's most successful Supernova search
program.
In the late 1990s it was proposed that recent supernova remnants could be found by looking for gamma rays from the decay of titanium-44.
This has a half-life of 90 years and the gamma rays can traverse the
galaxy easily, so it permits us to see any remnants from the last
millennium or so. Two sources were found, the previously discovered Cassiopeia A remnant, and the RX J0852.0-4622 remnant, which had just been discovered overlapping the Vela Supernova Remnant
In 1999 a star within IC 755 was seen to explode as a supernova and named SN 1999an.
This remnant (RX J0852.0-4622) had been found in front (apparently) of the larger Vela Supernova Remnant.
The gamma rays from the decay of titanium-44 showed that it must have
exploded fairly recently (perhaps around 1200 AD), but there is no
historical record of it. The flux of gamma rays and x-rays indicates
that the supernova was relatively close to us (perhaps 200 parsecs or
600 ly). If so, this is a surprising event because supernovae less than
200 parsecs away are estimated to occur less than once per 100,000
years.
2000 to present
Cosmic lens MACS J1720+35 helps Hubble to find a distant supernova.
The "SN 2003fg"
was discovered in a forming galaxy in 2003. The appearance of this
supernova was studied in "real-time", and it has posed several major
physical questions as it seems more massive than the Chandrasekhar limit would allow.
First observed in September 2006, the supernova SN 2006gy, which occurred in a galaxy called NGC 1260 (240 million light-years away), is the largest and, until confirmation of luminosity of SN 2005ap
in October 2007, the most luminous supernova ever observed. The
explosion was at least 100 times more luminous than any previously
observed supernova, with the progenitor star being estimated 150 times more massive than the Sun. Although this had some characteristics of a Type Ia supernova, Hydrogen was found in the spectrum. It is thought that SN 2006gy is a likely candidate for a pair-instability supernova. SN 2005ap, which was discovered by Robert Quimby
who also discovered SN 2006gy, was about twice as bright as SN 2006gy
and about 300 times as bright as a normal type II supernova.
Host Galaxies of Calcium-Rich Supernovae.
On May 21, 2008, astronomers announced that they had for the first
time caught a supernova on camera just as it was exploding. By chance, a
burst of X-rays was noticed while looking at galaxy NGC 2770,
88 million light-years from Earth, and a variety of telescopes were
aimed in that direction just in time to capture what has been named SN 2008D. "This eventually confirmed that the big X-ray blast marked the birth of a supernova," said Alicia Soderberg of Princeton University.
One of the many amateur astronomers looking for supernovae, Caroline Moore, a member of the Puckett Observatory Supernova Search Team, found supernova SN 2008ha late November 2008. At the age of 14 she had been declared the youngest person ever to find a supernova.
However, in January 2011, 10-year-old Kathryn Aurora Gray from Canada
was reported to have discovered a supernova, making her the youngest
ever to find a supernova. Mr. Gray, her father, and a friend spotted SN 2010lt, a magnitude 17 supernova in galaxy UGC 3378 in the constellation Camelopardalis, about 240 million light years away.
In 2009, researchers have found nitrates in ice cores
from Antarctica at depths corresponding to the known supernovae of 1006
and 1054 AD, as well as from around 1060 AD. The nitrates were
apparently formed from nitrogen oxides
created by gamma rays from the supernovae. This technique should be
able to detect supernovae going back several thousand years.
On November 15, 2010, astronomers using NASA's Chandra X-ray Observatory announced that, while viewing the remnant of SN 1979C in the galaxy Messier 100, they have discovered an object which could be a young, 30-year-old black hole. NASA also noted the possibility this object could be a spinning neutron star producing a wind of high energy particles.
On August 24, 2011, the Palomar Transient Factory automated survey discovered a new Type Ia supernova (SN 2011fe)
in the Pinwheel Galaxy (M101) shortly after it burst into existence.
Being only 21 million lightyears away and detected so early after the
event started, it will allow scientists to learn more about the early
developments of these types of supernovae.
On 16 March 2012, a Type II supernova, designated as SN 2012aw, was discovered in M95.
On January 22, 2014, students at the University of London Observatory spotted an exploding star SN 2014J
in the nearby galaxy M82 (the Cigar Galaxy). At a distance of around
12 million light years, the supernova is one of the nearest to be
observed in recent decades.
Future
The
estimated rate of supernova production in a galaxy the size of the Milky
Way is about twice per century. This is much higher than the actual
observed rate, implying that a portion of these events have been
obscured from the Earth by interstellar dust. The deployment of new
instruments that can observe across a wide range of the electromagnetic spectrum, along with neutrino detectors, means that the next such event will almost certainly be detected.
The Large Synoptic Survey Telescope (LSST) is predicted to discover three to four million supernovae during its ten-year survey, over a broad range of distances.
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